Cellulose ether derivatives, in particular, carboxymethyl cellulose or hydroxyethyl cellulose, have been used in extensive applications, and also used as a starting material for production of other cellulose derivatives. Many methods for production of the cellulose ether derivatives have been conventionally reported.
Carboxymethyl cellulose (hereinafter occasionally referred to merely as “CMC”) has been used in various applications such as thickeners, dispersants, emulsifiers, protective colloid agents and, stabilizers. CMC has been industrially produced by a solvent method in which cellulose is treated and activated with a large amount of an aqueous alkali solution to produce an alkali cellulose (cellulose alkalization) and then the resulting alkali cellulose is dispersed in a water-containing organic solvent to react with a monohaloacetic acid. In the solvent method, a hydrophilic solvent such as isopropanol is used, so that side reactions between the solvent or water and the monohaloacetic acid inevitably occur. Owing to a poor reactivity of the monohaloacetic acid to cellulose, an excessive amount of the monohaloacetic acid must be used for the purpose of attaining a desired substitution degree thereof. In addition, the above method has such a large burden that an additional purification step is needed for removing neutralized salts derived from an excessive amount of the alkali used, hydroxymethyl acetic acid salts as by-products, etc., by washing or the like.
JP 9-176201A discloses the method for production of CMC in which the reaction of cellulose with a monohaloacetic acid is carried out while being kept under neutral to weakly basic condition in order to enhance a reaction efficiency of the monohaloacetic acid. However, the method is still unsatisfactory because the effective utilization efficiency of the monohaloacetic acid is as low as 58 to 65%.
On the other hand, hydroxyethyl cellulose has been extensively used as a compounding component such as a dispersant and a stabilizer which are compounded in paints, cosmetics, building materials, thickeners, adhesives, drugs or the like.
Hydroxyethyl cellulose has been generally produced by a similar method to that used for production of CMC in which cellulose is treated and activated with a large amount of an aqueous alkali solution to prepare an alkali cellulose (cellulose alkalization) and then the resulting alkali cellulose is reacted with ethyleneoxide as an etherifying agent.
In the cellulose alkalization step, in order to remove a surplus amount of alkali or water from an alkali cellulose produced in the step, very complicated procedures such as filtration and squeezing are needed.
However, even when subjected to filtration or squeezing, the alkali cellulose usually still contains residual water in an amount equal to or more than the amount of the alkali cellulose. Further, it is considered that in the alkali cellulose obtained by the cellulose alkalization step, a majority of hydroxyl groups contained in a molecule of the cellulose are present in the form of an alcoholate. In fact, the alkali cellulose contains an alkali in an amount of usually about 3 mol, at least 1 mol, per a glucose unit in a molecule of the cellulose.
Although a cellulose ether is obtained by adding an etherifying agent such as ethyleneoxide to the cellulose thus activated by cellulose alkalization, the above-mentioned residual water that still remains in an amount equal to or more than the amount of the alkali cellulose even after the cellulose alkalization is also reacted (hydrated) with ethyleneoxide as the etherifying agent, so that a large amount of by-products such as ethylene glycol are produced.
In addition, since the reaction is usually carried out in a slurried condition, the reaction rate of ethyleneoxide to the cellulose is low. Therefore, in order to enhance the reaction rate of ethyleneoxide to the cellulose, it is required that ethyleneoxide is used in an excessive amount. Further, ethylene glycol as a by-product is readily reacted with ethyleneoxide as the etherifying agent, resulting in production of a large amount of polyoxyethylene as a by-product. In addition, an excessive amount of alkali and a large amount of water which are present in the reaction system tend to cause not only increase in amount of the reaction product but also considerable increase in amounts of the by-products, which tends to result in gelling of the reaction system.
To solve the above problems, not only water but also various polar solvents may be added to the reaction system to effectively conduct the slurry reaction. For example, in JP 8-245701A and JP 6-199902A, there is described the method of separating and recovering the solvent from a water phase of the reaction system after the cellulose alkalization and the reaction with the etherifying agent, by adding thereto a polar solvent that is hardly compatible with water such as tert-butanol and methyl isobutyl ketone in the reaction. However, unless any measure for considerably reducing the amounts of the alkali and water is taken, it is substantially difficult to reduce the amounts of the by-products such as ethylene glycol to a large extent.
Also, since the use of ethyleneoxide as the etherifying agent is strictly limited by the safety standard for high-pressure gases, there are many limitations to facilities used in the above method when industrially practiced.
Therefore, from the industrial viewpoints, it is useful to develop a process for producing cellulose ether derivatives in a convenient and efficient manner with a less amount of wastes.